The modification of silica gels for catalytic activity is known as discussed in Burwell, Chemtech, pp. 370-377 (1974).
The use of chromium compounds in the polymerization of olefins is also well-known. U.S. Pat. Nos. 2,825,721 and 2,951,816 teach the use of CrO.sub.3 supported on an inorganic material such as silica, alumina or combinations of silica and alumina, and activated by heating in reducing atmospheres, to polymerize olefins. When, however, a catalyst system of this type is used in techniques such as the well-known particle-form process, the resins produced, while useful in many applications, are unsatisfactory for others because of a deficiency in certain properties such as melt index.
Attempts to improve the properties of polyolefins produced using supported, heat-activated chromium oxide catalysts have been made by adding various compounds to the supported chromium oxide prior to the heat activation thereof. For example, U.S. Pat. No. 3,622,522 shows that an alkoxide of gallium or tin may be added to supported chromium oxide prior to heat activation (and compares addition of aluminum isopropoxide unfavorably). U.S. Pat. No. 3,715,321 suggests adding a compound of a Group II-A or Group III-B metal to supported chromium oxide prior to heat treatment, whereas U.S. Pat. No. 3,780,011 discloses adding alkyl esters of titanium, vanadium or boron, and U.S. Pat. No. 3,484,428 discloses adding alkyl boranes to such a catalyst.
It is also known to utilize other chromium compounds as catalysts for the polymerization of olefins. Such compounds include various silyl chromate and polyalicyclic chromate esters as described, for example, in U.S. Pat. Nos. 3,324,095; 3,324,101; 3,642,749; and 3,704,287. The use of phosphorus-containing chromate esters in olefin polymerization catalysts has also been disclosed in the aforesaid U.S. Pat. No. 3,704,287; and in U.S. Pat. No. 3,474,080. Other such catalysts are described in copending and commonly assigned application Ser. No. 532,131 filed Dec. 16, 1974.
Use of the above chromium compound catalysts in Ziegler-type coordination catalyst systems has also been proposed. As is well-known in the art, such catalysts frequently additionally comprise organometallic reducing agents such as, for example, trialkyl aluminum compounds. Ziegler-type catalyst systems incorporating supported chromium compound catalysts and organometallic reducing agents, particularly organoaluminum compounds, are disclosed, for example, in U.S. Pat. Nos. 3,324,101; 3,642,749; 3,704,287; 3,806,500; and in the aforesaid copending application Ser. No. 532,131.
It has also been disclosed, in copending application Ser. No. 558,504 filed Mar. 14, 1975, owned by the assignee of the present invention, that the properties of olefin polymers, e.g., melt indexes, may be substantially improved by the use of a catalyst prepared by depositing chromium and aluminum compounds on an inorganic support material and heat-activating the supported composition in a non-reducing, preferably oxygen-containing atmosphere, at a temperature of from about 300.degree. C. (572.degree. F.) up to the decomposition temperature of the support. The resulting material, as preferably combined with a metallic and/or non-metallic reducing agent, e.g., a trialkyl borane, provides a catalyst system capable of producing polymers having improved flow properties and shear responses in addition to increased melt indexes.
It is among the objects of the present invention to provide yet a further improved technique for preparing catalytic materials, especially olefin polymerization catalysts, and a polymerization process utilizing the same which results in the formation of polymers having increased melt indexes, and improved flow properties, shear responses and other desired characteristics.